A typical fuel cell system includes a power section in which one or more fuel cells generate electrical power. A fuel cell is an energy conversion device that converts hydrogen and oxygen into water, producing electricity and heat in the process. Each fuel cell unit may include a proton exchange member (PEM) at the center with gas diffusion layers on either side of the proton exchange member. Anode and cathode catalyst layers are respectively positioned at the inside of the gas diffusion layers. This unit is referred to as a membrane electrode assembly (MEA). Separator plates (also referred to herein and flow field plates or bipolar plates) are respectively positioned on the outside of the gas diffusion layers of the membrane electrode assembly. This type of fuel cell is often referred to as a PEM fuel cell.
The reaction in a single MEA typically produces less than one volt. Therefore, to obtain operating voltages useful in most applications, a plurality of the MEAs may be stacked and electrically connected in series to achieve a desired voltage. Electrical current is collected from the fuel cell stack and used to drive a load. Fuel cells may be used to supply power for a variety of applications, ranging from automobiles to laptop computers.
It is recognized that for certain applications, such as stacks used for automotive drives, there are limitations with existing PEM Fuel Cells due to excessive weight, volume, and cost. One reason for this is due to the thickness and weight of the flow field separators. Machined graphite, carbon composite, and metals are materials commonly used for flow field separators. These material options may suffer from either excessive volume or weight. This limitation leads to heavy or bulky fuel cell stacks, as typically there are many separators in each stack. Furthermore, it is difficult to make these separators thin and robust. Breakage and cracking have been issues with graphite and carbon composite based separators. Small defects can lead to breakage and catastrophic failures. Thin, light weight metal plate separators can bend easily and remain deformed. There have been many attempts to improve the performance of flow field separators, but it has been difficult to find a good balance between cost, thickness, weight, and toughness.
Even where the thickness of the flow field separators can be reduced, there are still space constraints in some applications that make it difficult to adapt fuel cells to practical designs. For example, some electric drive motors used in automobile applications may require electrical potentials as high as 100 volts or more. In order for a fuel cell system to provide this potential without expensive power conversions, the fuel cell stack would require a large number of MEAs stacked together, making the fuel cell stack larger than desirable.
Other design requirements limit how compact a fuel cell system can be. For example, gases and fluids need to flow through the stack in order to power the cells and to regulate the cell temperature. The internal flow passages and external plumbing needed to accommodate these gases and fluids may make it difficult to produce a fuel cell assembly that is easy to integrate in a space-constrained environment such as an automobile. However, the potential benefit resulting from practical, fuel cell powered automobiles is great, so cost effective and robust solutions to these limitations are desirable.